1. Field of the Invention
The present invention relates in general to an image forming device using liquid developer and control method thereof. More particularly, the present invention relates to a detector for detecting an abnormality in a developer passage between a developer cartridge and a developing device, through or along which a liquid developer from the developer cartridge is supplied and collected, to and from the developing device during a developing process, an image forming device having the same and a detection method thereof.
2. Description of the Related Art
In general, an image forming device using a liquid developer, such as a wet electrophotographic printer, includes a developer supply system for supplying the liquid developer to a developing device for use in an image formation process. There are two types of developer supply systems: a combination type, where a developer cartridge and a developing device are combined in one body, and a split type, where a developer cartridge and a developing device are connected by a connecting tube.
The split-type developer supply system typically employs a developer supply pump and/or a developer recovery pump during the developing process to supply liquid developer from the developer cartridge to the developing device through the connecting tube. The split-type developer supply system then recovers the developer from the developing device to the developer cartridge.
FIG. 1 illustrates a conventional electrophotographic printer 10 based on the split-type developer supply system. The wet electrophotographic printer 10 includes an image forming unit 11, and a developer supply unit 21. The image forming unit 11 includes a photosensitive drum 1, such as an organic photoconductive drum (OPC), a laser scanning unit 33, a charger 6, a discharger 7, a developing device 26, and a cleaning blade 8. These constituents interface with one another to perform the image forming procedure in sequence, including the charging, discharging, exposing, and developing operations, thereby forming a desired image on the photosensitive drum 1.
The developing device 26 includes a developing chamber retaining a liquid developer 48, a developing roller 2 disposed under the photosensitive drum 1, a deposit roller 5 applying an electric force to the liquid developer 48 thereby forming a charged developer layer on the developing roller 2, and a metering roller 3 that regulates the charged developer layer formed on the developing roller 2. The metering roller 3 regulates the charged developer layer formed on the developing roller 2 by means of the deposit roller 5 to a developer layer with a fixed amount of toner or concentration (% solid). The metering roller 3 also supplies the developer layer to a nip between the developing roller 2 and the photosensitive drum 1. The developing device 26 also includes a cleaning roller 4 for cleaning the developing roller 2.
To supply the liquid developer 48 to the developing chamber 27, the developer supply unit 21 is disposed over the developing device 26. The developer supply unit 21 includes a developer cartridge 15 that forms a developer storage chamber, and is provided with a first supply inlet 16 and a first developer outlet 17. The developer supply unit 21 further includes a developer supply pump 18 disposed between the first developer outlet 17 of the developer cartridge 15 and a second developer inlet 13 of a developing chamber 27 through first and second connecting tubes 22, 23, and a recovery pump 19 disposed between the first developer inlet 16 of the developer cartridge 15 and a second developer outlet 14 of a recovery chamber 28 through third and fourth connecting tubes 24, 20.
The developer supply pump 18 and the developer recovery pump 19, respectively, includes a first and a second direct current (DC) motor 14, 16 rotating at predetermined speeds under a first and a second pulse width modulated (PWM) signal PWM1, PWM2, each with a predetermined PWM duty cycle value generated from a controller 29.
Operation of the wet electrophotographic printer 10 in regard to the structure discussed above will now be explained. When a print command is applied, the discharger 7 discharges an electric potential on the surface of the photosensitive drum 1, and an electrostatic latent image, or a charged layer corresponding to a print image, is formed on the photosensitive drum 1 by means of the charger 6 and the laser scanning unit 33. Then, a developer layer with a large amount of toner is adhered onto the electrostatic latent image to form a toner image. The developer layer is formed on the developing roller 2 using the liquid developer 48 in the developing chamber 27 by means of the deposit roller 5 and the metering roller 3.
The developer supply pump 18 is driven by a driving voltage supplied to the first DC motor 14 from a power source (not shown) under control of the first PWM signal PWM1 from the controller 29, and supplies the liquid developer 48 in the developer cartridge 15 to the developing chamber 27. The developer recovery pump 19 is driven by a driving voltage supplied to the second DC motor 16 from the power source under the control of the second PWM signal PWM 2 from the controller 29 and recovers liquid developer 48 in the recovery chamber 28 that has overflowed from the developing chamber 27 to the developer cartridge 15.
Concurrently, the toner image that was developed on the photosensitive drum 1 by means of the developer 26 is transferred onto a sheet of print paper with the assistance of a transfer belt (not shown), and a transfer roller (also not shown). The toner image is fixed onto the sheet of print paper by a fixing roller (not shown) to form a final image.
Inevitably, portions of the liquid developer are not transferred to the transfer belt, but remain on the photosensitive drum 1. This residual developer is recovered to a waste developer reservoir 12 by the cleaning blade 8. The photosensitive drum 1 is ready then to be subject to the above-described procedure again for the formation of the next electrostatic latent image and developing of the desired image thereon, with the aid of the discharger 7, the charger 6, the laser scanning unit 33, and the developing device 26.
After the printer 10 completes the printing process, the developer supply pump 18 rotates in the opposite direction of the operation direction when the print command is first applied. This occurs because the driving voltage that it supplied to the first DC motor 14 from the power source is controlled to the opposite polarity by the first PWM signal PWM1 from the controller 29. As a result, the residual liquid developer 48 in the developing chamber 27 is recovered to the developer cartridge 15, and the printing process is ended.
Structural problem exist, however, in the above-described printer 10. The developer passage in which the liquid developer is supplied and recovered between the developer cartridge 15 and the developing device 26 sometimes leaks or becomes clogged. This condition is often not detected by the printer 10.
In particular, when the developer supply pump 18 and the developer recovery pump 19 operate to supply or recover the liquid developer 48 in the developer cartridge 15 to or from the developing device 26, any or all of the developer passage, the developer supply pump 18 and the developer recovery pump 19, might have a leak. The developer passage consists the first and second developer inlets 16, 13, the first and second developer outlets 17, 14, the first, second, third, and fourth connecting tubes 22, 23, 24, 20, and the space among the first, second, third, and fourth connecting tubes 22, 23, 24, 20. The leak can be caused by a manufacturing and/or assembly defect, as a result of a build-up of sediment of the developer sludge, or the sediment clogging of the liquid developer 48.
When the liquid developer 48 leaks, the peripheral components inside the printer are all contaminated by the liquid developer. In addition, when the liquid developer 48 leaks or the passage thereof is stopped up, the developer supply pump 18 and/or the developer recovery pump 19, each being driven by the first and second DC motors 14, 16 whose rotational speeds vary in accordance with the load of the liquid developer 48, can change the speed of rotation.
Further, if the rotational speed of the developer supply pump 18 and/or the developer recovery pump 19 is changed, the amount of the liquid developer 48 to be supplied into the developing chamber 27 and/or the amount of the liquid developer 48 recovered from the recovery chamber 28 varies from the predetermined amounts. Accordingly, the level of the liquid developer 48 in the developing chamber 27 and/or the recovery chamber 28, particularly in the developing chamber 27, changes. As a result, the concentration of the liquid developer formed on the developing roller 2 by the deposit roller 5 and the metering roller 3 changes, and the final image quality deteriorates.
Moreover, because of the leak in the developer passage or the clogged-up passage, if the developer recovery pump 19 runs at a much lower speed than the developer supply pump 18, or if the developer supply pump 18 runs at a much higher speed than the developer recovery pump 19, the amount of the liquid developer 48 supplied through the developer supply pump 18 is greater than the recovered amount of the liquid developer 48 through the developer recovery pump 19. In such case, the liquid developer 48 overflows the recovery chamber 28, thereby contaminating the surroundings.
Therefore, there is a need to develop a detector for detecting an abnormality, such as leakage or clogging, in the developer passage, through which the liquid developer 48 passes to and from the developing device 26.